Render glow over transparent background blender 3D evee
Okay, I am not good with compositing but maybe will be able to help you a bit. It is a bit cheatish way to create a false bloom using gaussian blur.
This should work with both cycles and eevee, my example is in eevee, which does not support lighting from emission material.


First a basic scene with sphere, cube and transparent background:
What we want to do is to make the cube glow. We will do it like this:


Add a new view layer, so we have two view layers.
Sphere will be excluded from one view layer, cube will be excluded from another, each of them will have their own view layer.

Lets add view layer:

Lets add collection and put cube into that collection

Set viewlayer to our first layer (defaultly named View Layer), select Collection 2 and exclude it from the layer:

Set view layer to the one we have added in step 3:

Select first collection (one with sphere, light and camera), right click it and exclude it from currently selected view layer:



What we've done so far is set two view layers, have cube in collection 2 included in View Layer.001, and Camera, Sphere and Light included in View Layer.


Now lets add emission material to our cube:

Open Compositing window, turn on Use Nodes, Auto Render and Backdrop. Two last options make preview of the render be rendered in the background whenever you change something in compositing nodes. As of this moment it seems to be requiring you to make one render before it starts working (just press f12 to render and then close rendered image).

What we are doing now is splitting image into two Render Layers, one for View Layer (with sphere) and other for View Layer.001 for Cube. Cube is blurred to create lighting aura effect and then added to cube (so we get a solid cube with blurred cube behind it). Then we add sphere from view Layer on top of that. Connecting to Composite node causes it to be renders output, Viewer node is required for Backdrop to display.

And final render: